Crop to EXACT aspect specified

If I specify an aspect ration of 223×120 when cropping an image, often I wind up with 223×118 or 223×119. In my case, this is a problem. Is there a way to ensure that I always get a thumbnail that is precisely what I specify?

I know I can set uploads to automatically crop to the exact thumbnail size, but I need to be able to control the cropped area instead of just some random rectangle in the middle of the image.